1. Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging (MRI), in particular to an imaging method and device for water/fat separation in magnetic resonance imaging, and also relates to a machine-readable storage medium and a computer program.
2. Description of the Prior Art
In magnetic resonance imaging (MRI), hydrogen protons in fat tissue in the human body and hydrogen protons in other tissues have different molecular environments, and as a result have different resonant frequencies. When hydrogen protons in fat and other tissue are simultaneously excited by an RF pulse, the relaxation times thereof are also different. Signals are acquired at different echo times, and fat tissue and non-fat tissue display different phases and signal strengths.
The Dixon method is a method for generating a pure water proton image in MRI. The basic principle thereof is to separately acquire two types of echo signals, in-phase and opposed-phase, for water and fat protons; the two types of signals with different phases undergo arithmetic operation, each producing a pure water proton image and a pure fat proton image, thereby achieving the objective of fat suppression. There are currently many Dixon imaging methods for water/fat separation in the art, including: the single-point Dixon method, the two-point Dixon method and the three-point Dixon method, etc. In a turbo spin echo (TSE; also called a fast spin echo, FSE) pulse sequence based on the Dixon method, the radio frequency (RF) pulse sequence comprises a 90° excitation pulse and 180° refocusing pulses (also called rephasing pulses); multiple echoes can be acquired between two adjacent refocusing pulses (i.e. in one echo interval). Different echoes corresponding to the same refocusing pulse use the same phase encoding, and the echoes corresponding to corresponding positions of different refocusing pulses form an echo set; for example: the first echo appearing after refocusing pulse 1, the first echo appearing after refocusing pulse 2, . . . , the first echo appearing after refocusing pulse n could form one echo set, while the second echo appearing after refocusing pulse 1, the second echo appearing after refocusing pulse 2, . . . , the second echo appearing after refocusing pulse n could form another echo set. One image can be reconstructed from each echo set independently. Since different echoes carry different amplitude and phase information, further calculation allows a pure water image and a pure fat image to be reconstructed separately.
FIG. 1 shows an FSE sequence based on the two-point Dixon method. Single-pole symmetric readout gradients are used, i.e. the readout gradients have the same polarity, and have a symmetric shape with the center of an echo as the axis of symmetry. The echo signals acquired are as shown in formula (1) below:Sτ=W+Fexp(jfΔτ)  (1)
Here, Sτ is the signal obtained for the echo offset τ, W is the water signal from the object under test, F is the fat signal from the object under test, the difference between the resonant frequencies of fat and water is represented by fΔ with units of Hertz (Hz), and j is the square root of −1.
In FIG. 1, τ=0 at the center of each echo interval, and the spin echo sequence requirement is met; a conventional spin echo image can be obtained by means of echo 12 and echo 22, the signals acquired for echo 12 and echo 22 being expressed as formula (2) below:S0=W+F  (2)
With regard to echo 11 and echo 21, the echo centers thereof are offset with respect to the center of the echo interval, and when τ is delta=−½fΔ and F and W have a phase difference of 180°, the signals acquired for echo 11 and echo 21 are expressed as formula (3) below:Sdelta=W−F  (3)
Clearly, by processing the echo signals in accordance with formulas (2) and (3), it is possible to work out the water signal W and the fat signal F. Two sets of echoes can be obtained using the FSE sequence based on the two-point Dixon method shown in FIG. 1. Suppose that there are n refocusing pulses; then one set comprises echo 11, echo 21, echo n1, while the other set comprises echo 12, echo 22, . . . , echo n2. By processing the signals from these two sets of echoes separately, two images can be reconstructed: one is an image in conformity with formula (2), with water and fat in phase, while the other is an image in conformity with formula (3), with water and fat in opposed phases. By subjecting these two images to a water/fat separation algorithm, a pure water image and a pure fat image can be obtained separately.
In the FSE sequence shown in FIG. 1, a reverse rephasing gradient must be inserted between two single-pole readout gradients in each echo interval, such as rephasing gradient 1 and rephasing gradient 2 in FIG. 1. Each readout gradient is divided by the center of the echo into two symmetric parts, which may be referred to as the front part and rear part of the readout gradient, and the moment of the rephasing gradient must be equal to the sum of the moments of the rear part of the preceding readout gradient and the front part of the subsequent readout gradient. In a drawing of an FSE sequence, the moment of a gradient can be characterized by the area of the gradient (the area being related to the gradient duration, rate of change of the gradient, and the amplitude, etc.). In other words, the area of the rephasing gradient is equal to the sum of the areas of the rear part of the preceding readout gradient and the front part of the subsequent readout gradient. Therefore the amplitude of the rephasing gradient will be higher than that of the readout gradients. However, in a low-field scanner (also called a low-field magnetic resonance system), gradients of high amplitude will generally cause serious problems in the form of eddy currents and accompanying fields, leading to a serious problem of artifacts (such as fuzziness, ghosts and shadows) in the reconstructed image. In particular, this problem with artifacts caused by eddy currents and accompanying fields will be more pronounced when an FSE sequence with a longer echo chain is used (i.e. a greater number of echoes appear during one excitation pulse cycle (TR)).
FIG. 2 shows an example of an image obtained by a low-field scanner using the FSE sequence shown in FIG. 1 and T2-weighted imaging. In FIG. 2, the left half shows extreme fuzziness of a water/fat separated image in a neck imaging application, while the right half shows a shadow which has appeared as a result of extreme non-uniformity of signal strength in phantom imaging.
It can be seen from the above that imaging technology for water/fat separation based on the two-point Dixon method in the prior art does not give ideal imaging results. The problem of eddy currents and accompanying fields caused by rephasing gradients of high amplitude is in urgent need of a solution, in order to mitigate the problem of artifacts in MRI.